The production of integrated circuits begins with the creation of high-quality semiconductor wafers. During the wafer fabrication process, the wafers may undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high-precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. As the size of the integrated circuits continues to decrease and the density of microstructures on an integrated circuit increases, the need for precise wafer surfaces becomes more important. Therefore, between each processing step, it is usually necessary to polish or planarize the surface of the wafer to obtain the flattest surface possible.
For a discussion of chemical mechanical planarization (CMP) processes and apparatus, see, for example, Arai, et al., U.S. Pat. No. 4,805,348, issued February, 1989; Arai, et al., U.S. Pat. No. 5,099,614, issued March, 1992; Karlsrud et al., U.S. Pat. No. 5,329,732, issued July, 1994; Karlsrud, U.S. Pat. No. 5,498,196, issued March, 1996; and Karlsrud et al., U.S. Pat. No. 5,498,199, issued March, 1996.
Such polishing is well known in the art and generally includes attaching one side of the wafer to a flat surface of a wafer carrier or chuck and pressing the other side of the wafer against a flat polishing surface. In general, the polishing surface comprises a horizontal polishing pad that has an exposed abrasive surface of, for example, cerium oxide, aluminum oxide, fumed/precipitated silica or other particulate abrasives. Polishing pads can be formed of various materials, as is known in the art, and which are available commercially. Typically, the polishing pad may be a blown polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation in Scottsdale, Ariz. The hardness and density of the polishing pad depends on the material that is to be polished.
During the polishing or planarization process, the workpiece or wafer is typically pressed against the polishing pad surface while the pad rotates about its vertical axis. In addition, to improve the polishing effectiveness, the wafer may also be rotated about its vertical axis and oscillated back and forth over the surface of the polishing pad.
Also, during the polishing of planarization process, a slurry solution is typically introduced onto the polishing surface, adding the chemical component to the chemical mechanical planarization ("CMP") process. Various different types of slurry solutions may be used to polish the semiconductor wafers, most of which are available commercially from various sources, such as, for example, Rodel, and Cabot. The type of slurry used generally depends on the material layer of the wafer being polished. For example, one type of slurry may be used to polish a bare silica surface of a wafer, while other types of slurries may be used to polish an oxide dielectric layer or a metal conductor layer. Moreover, deionized water is very often introduced into the system for cleaning purposes.
One of the more costly aspects of the wafer polishing process surrounds the consumption and disposal of the slurry compounds and the deionized water. It is therefore advantageous to precisely monitor and control the amount of slurry and deionized water used during the semiconductor wafer fabrication process. However, given the nature of the slurry compounds, no device to date has been found that accurately and clearly measures slurry flow. For example, standard flywheel and rotor type meters are inadequate because the fine abrasive particles in the slurry solutions tend to damage and jam the mechanical components of those meters. In addition, as the abrasive particles wear on the mechanical components of the meters, particulates worn from the components can contaminate the slurry, causing extreme damage to the wafers. Therefore, a noncontacting fluid sensing device is considered desirable.
Various types of noncontacting fluid sensing devices are generally known in the art, including electromagnetic flow meters, ultrasonic flow meters, thermal dispersion flow detectors and meters, vortex shedding meters, and rotameters with Hall effect electronic transducers, to name a few. However, most are inadequate for measuring the slurry flows in a CMP environment.
Electromagnetic flow meters are typically used in the mining industry to detect the large slurry flows used in that industry. As such, the electromagnetic flow meters are generally too large to be used with the CMP machines. Ultrasonic flow meters, like electromagnetic meters, are also too large for practical use with CMP machines. In addition, ultrasonic flow meters need to be fully charged with fluid in order to operate. This condition greatly affects the accuracy of the meter at the beginning and end of slurry flow cycles. Thermal dispersion flow detectors and meters have very slow response times, typically fifteen to twenty-five seconds; this is far too slow to accurately measure slurry flow rates in CMP machines. In addition, thermal dispersion flow detectors and meters are ineffective when used to detect the low slurry flow amounts that exist in CMP machines. Similarly, the vortex shedding meters also cannot adequately detect the low flow ranges of the CMP machines. Finally, the rotameters with Hall effect electronic transducers are inadequate because the meter's electronics are unstable, and the meter's sensors are ultra-sensitive to any metallic object they come in contact with. For example, the sensor can waiver up to ten percent (10%) when a metal object passes close to it. This condition is totally inadequate for use on a CMP machine where metal parts are prevalent.
A system is thus needed which accurately measures the flow rates of slurry and deionized water to the CMP machine and controls the flow rate during the CMP process which overcomes the shortcomings of the prior art.